The use of high purity 2,2-bis(4-hydroxyphenyl) propane, herein referred to as bisphenol-A, as a reactant in the preparation of subsequent formulations such as in the preparation of polycarbonate resins is well known in the art. One method for obtaining the purity needed of the bisphenol-A is to distill crude bisphenol-A. In such distillations, various by-products remain and these primarily include higher condensation products of bisphenol-A, condensation products of phenol and acetone produced in the original bisphenol-A formation, colored substances, isomers of bisphenol-A, and the like. Prahl et al, U.S. Pat. No. 3,290,390, disclose the addition of phenol to the by-products and contacting this resulting mixture with an acidic agent such as hydrogen chloride at between room temperature and 150.degree. C. to produce therefrom bisphenol-A. However, the method of Prahl et al requires, according to their examples, 16 hours or more reaction time.
It has now been discovered that it is possible to convert such by-products isomerizable to bisphenol-A to said bisphenol-A in a vastly shorter period of time by treating a mixture of the by-products and phenol with anhydrous hydrogen chloride under superatmospheric pressure. The process of this invention yields bisphenol-A in solution for a subsequent recovery such as by cooling the solution to produce a 1:1 bisphenol-A/phenol adduct, from which the bisphenol-A can then be recovered, by procedures known per se.
In practice, commercial plants utilize distillation of bisphenol-A as a step in the purification process. The bottoms from the distillation are called "tars", and conventionally these are disposed of by burning. This represents a serious loss in yield because tars contain from 20-60% bisphenol-A plus the isomerizable by-products mentioned above. Experiments have shown that the amount of bisphenol-A in a typical tar can be more than doubled by isomerization with HCl at 45 psig and more than 70% of this bisphenol-A can be recovered, e.g., by crystallization. The mother liquor from the crystallization can be stripped of phenol (for recycle) in a separate column, and the diminished quantity of "tar" remaining can be burned or otherwise utilized.